A cellulose ester film has mainly been used as a support of recording materials such as a photographic material. Examples of cellulose ester include cellulose acetate, cellulose acetate butylate and cellulose acetate propionate. Cellulose acetate film (particularly cellulose triacetate film) has been widely used because it has excellent properties such as dimensional stability, transparency and gloss.
The cellulose ester film generally has low mechanical strengths such as tearing strength, folding endurance and impact strength. Particularly at low temperature, the film is very hard, brittle and easy to be teared.
It has been well known that a monomeric plasticizer is added to the film to improve the mechanical strengths. Examples of the monomeric plasticizer include triphenyl phosphate, tricresyl phosphate, octyl diphenyl phosphate, triethyl phosphate, tributyl phosphate, diethyl phthalate, dimethoxyethyl phthalate, ethylphthalyl ethyl glycolate, butyl phthalyl butyl glycolate and triacetin. A phosphoric ester such as triphenyl phosphate has mainly been used for cellulose triacetate film.
The preparation for preparation of the cellulose ester film generally comprises the steps of dissolving the cellulose acetate and a plasticizer in a solvent to prepare a dope, casting the dope on a support and evaporating the solvent. Methylene chloride is mainly used as the solvent. The support usually is a drum or a band which has a polished surface. For example, the film can be prepared by casting a specific dope on a support at a temperature of not more than 10.degree. C., exposing the dope to blowing air for at least 2 seconds, peeling off and drying the resulting film at a high temperature (100.degree. to 160.degree. C.) to evaporate the remaining solvent to obtain a film (cf., Japanese Patent Provisional Publications No. 60(1985)-176834 and No. 60(1985)-203430).
In the above-mentioned process, the monomeric plasticizer tends to be evaporated with the remaining solvent. The evaporated plasticizer induces contamination at the drying step. Further, the plasticizer migrates vertically so that the distribution of the plasticizer tends to be heterogeneous. These problems are very serious because they cause curling of the film and unsatisfactory adhesion between the film and a coated layer provided thereon. In order to solve these problems, polymeric plasticizers have been proposed (cf., Japanese Patent Publications No. 43(1968)-16305, No. 44(1969)-32672 and No. 47(1972)-760). Examples of the polymeric plasticizers include specific polyurethanes polyesters and copolymers thereof which are soluble in methylene chloride.
The polymeric plasticizers can be used to solve the problems caused by the migration and volatilization of the plasticizer. Further, the polymeric plasticizers much enhance the mechanical strengths (such as the tearing strength and the folding endurance) of film at a low temperature. However, the films using the polymeric plasticizers exhibit large dimensional variation depending on humidity. A large dimensional variation induces expansion or shrinking of intervals between perforations, and causes serious troubles such as a projection hitch and shift of printing, particularly in the case that the film is used as a photographic support. Further, the films using the polymeric plasticizers are very combustible. The combustibility is a serious problem when the cellulose ester film is used. For example, the combustibility of a movie film is strictly defined in Japanese Industrial Standard. Moreover, when the film is used as a support of photographic material, the plasticizer should not affect photographic properties.
Japanese Patent Provisional Publication No. 60(1985)-250053 proposes using a dimer of a halogenated alkylphosphoric ester as a plasticizer to solve the above-mentioned problems. The problems caused by the migration and volatilization of the monomeric plasticizer are also solved by this plasticizer. However, the mechanical strengths of the film such as the tearing strength and the bending strength are still insufficient, and desired to be more improved.